The present invention relates to a semiconductor device in which operation functions can be changed over through switches.
A one-chip micro computer generally has a configuration in which a central processing unit (hereinafter referred to as "CPU"), a read only memory (hereinafter referred to as "ROM"), a random access memory (hereinafter referred to as "RAM") and an input/output device (hereinafter referred to as "I/O") are formed on one and the same semiconductor substrate. In recent years, this type one-chip micro computer has been required to have a large number of functions. On the other hand, this type one-chip micro computer has been required to have various optional functions in accordance with the diversification of interfaces with peripheral devices and the diversification of specifications demanded by users. As a method in which optional functions capable of being changed over through certain switches are incorporated in this type one-chip micro-computer, there is a method in which registers corresponding to such optional functions are provided so that the functions can be switched by changing data of the registers. In this method, however, it is necessary that registers and address decoders are provided correspondingly to the large number of optional functions. Accordingly, this type one-chip micro computer must have a large number of registers unnecessarily, so that increase of chip size and increase of chip cost cannot be avoided. Upon such circumstances, conventionally, such optional functions as used after fixed by users are formed simultaneously in the same semiconductor device producing process in which data of a program ROM are formed.
On the other hand, in order to develop data of the program ROM for this type one-chip micro computer and in order to evaluate the data in the condition that the program ROM is mounted on this type one-chip micro-computer, a PROM built-in micro computer formed by replacing-the ROM by a PROM is used as a test IC. In such a PROM built-in micro computer, there may arise a disadvantage in that the operation of a test IC is different from the operation of a real IC, because it is impossible to switch the above-mentioned optional functions in the case where the functions are provided in the form of mask options. Further, in the present state, such a PROM built-in micro computer formed by providing a mask ROM in the form of a PROM is indispensable to perform development of program data of the mask ROM type.
As described above, the PROM built-in micro computer is mainly used for development. Accordingly, in a one-chip micro computer including a plurality of mask options, it is important that compatibility between the mask ROM type and the PROM type is secured for development of program data. Accordingly, in the PROM type, it is necessary that the mask options are provided in the form of a PROM. The mask options are however arranged dispersively in the IC. Accordingly, in a general method in which data are written in a PROM so that one data is allocated to one address correspondingly, increase of chip size cannot be avoided because address signals, data signals, address decoder signals and write control signals are drawn around within the chip.